The design of high performance radio frequency (RF) transmitters may use phase-locked frequency synthesizers that experience transmission delays of hundreds of milliseconds to lock onto the desired transmit frequency when the transmitter is turned on.
The delay problem may be solved by leaving the RF frequency generator running continuously, rather than intermittently turning it on and off. With the frequency generator running continuously, the transmitter remains phase-locked upon the proper RF frequency. Thus, the proper RF carrier frequency is always available for instantaneous transmission. However, leaving the RF generator running continuously has its own attendant problems.
One of the problems experienced in leaving the RF generator running is the existence of undesirable signal feed-through. Unwanted signal propagation, such as feed-through, is generally controlled through the amplifying stage by turning off the active amplifying device. Various methods for disabling the active amplifying device include removal of collector voltage, removal of base bias voltage, reverse biasing the base-emitter junction, or simply removing the input signal, in class C amplifiers. However, during the standby state in a radio transmitter, parasitic capacitances of bipolar amplifying devices result in the coupling of significant amounts of RF energy between the inputs and outputs of amplifier stages. In common-emitter amplifier stages, parasitic collector-base capacitance allows significant coupling of energy between the input and output, resulting in undesirable signal feed-through.
Other methods to stop unwanted signal propagation may involve the use of relays or diodes to short or open circuit the input or output of amplifier stages.
A conventional solution to the signal feed-through problem is to shunt undesirable signals by using PIN diodes. Disadvantageously, the PIN diodes require separate biasing networks. While a transmitter is actively transmitting, the shunt PIN diode is biased to present a relatively high impedance. During standby, the PIN diode is rebiased to present a low impedence, thereby shunting undesirable signals.
All of these previous methods for improving feed-through attenuation use devices external to the active amplifying devices themselves. Each external device utilized has its own negative attributes such as additional cost, signal losses, energy losses, and additional components for impedance matching.
It is an object of the present invention to provide a simple, yet effective, solution for the problem of undesirable signal feed-through in RF amplifier stages.
It is an object of the present invention to minimize the number of external devices utilized for attenuating undesirable signals.
It is a further object of the invention to minimize the number of components required and their associated cost, signal losses, and energy losses.
The ultimate object of the invention is to provide greatly improved feed-through attenuation by utilizing the active amplifying device itself as the attenuator.
These and other objectives are accomplished by the present invention.